7.2     Review of Lewis Acids and Bases

7.2a The Curved Arrow Formalism Recall that a most important bookkeeping technique, called the curved arrow formalism (p. 24), helps to sketch the broad outlines of what happens during a chemical reaction. However, it does not constitute a full reaction mechanism, which is a description of how the reaction occurs (p. 136). If a reaction mechanism is not an arrow formalism, what is it? A short answer would be a description, in terms of structure and energy, of the intermediates and transition states separating starting material and product. Implied in this description is a picture of how the participants in the reaction must approach each other. The arrow formalism is a powerful device of enormous utility in helping to keep track of bond makings and breakings. In this formalism, chemical bonds are shown as being formed from pairs of electrons flowing from a donor toward an acceptor, often, but not always, displacing other electron pairs. Figure 7.3 shows two examples. In the first, hydroxide ion, acting as a Brønsted base, displaces the pair of electrons in the hydrogen–chlorine bond. The reaction of ammonia (:NH₃) with hydrogen chloride is another example in which the electrons on neutral nitrogen are shown displacing the electrons in the hydrogen–chlorine bond.

7.2b Lewis Acids and Bases In Sections 1.8 and 2.15, we began a discussion of acids and bases that will last throughout this book. Then we spent some time in Chapter 3 reviewing Brønsted acids and bases in the context of the addition reactions and extended the subject of acidity and basicity to Lewis acids and Lewis bases. These concepts will hold together much of the material in the remaining chapters.

Consider two conceptually related processes: the reaction of boron trifluoride with fluoride ion, F⁻, and the reaction of the methyl cation with hydride, H:⁻ (Fig. 7.4).

In these two reactions, no proton is being donated or accepted, so by a strict definition of Brønsted acids and bases we are not dealing with acid–base chemistry. As we already know from Chapter 3, however, a more general definition of acids and bases does deal with this situation. A little review may be helpful: A Lewis base is defined as any species with a reactive pair of electrons (an electron donor). This definition may seem very general, and indeed it is. It gathers under the “base” category all manner of odd compounds that don’t really look like bases. The key word that confers all this generality is reactive. Reactive under what circumstances? Almost everything is reactive under some set of conditions. And that’s true; compounds that are extraordinarily weak Brønsted bases can still behave as Lewis bases.

If the definition of a Lewis base seems general and perhaps vague, the definition of a Lewis acid is even worse. A Lewis acid is anything that will react with a Lewis base (thus, an electron acceptor). Therein lies the power of these definitions. They allow us to generalize—to see as similar reactions those that on the surface appear very different.

To describe Lewis acid–Lewis base reactions in orbital terms, we need only point out that a stabilizing interaction of orbitals involves the overlap of a filled orbital (Lewis base) with an empty orbital (Lewis acid) (Fig. 7.5).

 

In the examples in Figure 7.4, the Lewis bases are the fluoride and hydride ions. In fluoride, the electrons occupy a 2p orbital, and in hydride they occupy a 1s orbital. The Lewis acids are boron trifluoride and the methyl cation, each of which bears an empty 2p orbital. A schematic picture of these interactions is shown in Figure 7.6.

7.2c HOMO–LUMO Interactions It is possible to elaborate a bit on the generalization that “Lewis acids (electrophiles) react with Lewis bases (nucleophiles)” by realizing that the strongest interactions between orbitals are between the orbitals closest in energy. For stabilizing filled–empty overlaps, this will almost always mean that we must look at the interaction of the highest occupied molecular orbital (HOMO) of one molecule (A), which is the nucleophile, with the lowest unoccupied molecular orbital (LUMO) of the other (B), which is the electrophile (Fig. 7.7).

In the reaction of BF₃ with fluoride, it is the filled fluorine 2p orbital that is the HOMO and the empty 2p orbital on boron that is the LUMO (Fig. 7.8). This nucleophile plus electrophile reaction forms a new B―F bond to produce BF₄⁻. All Lewis acid–Lewis base reactions can be described in similar terms.